This invention relates to pneumatic spring devices and more particularly to those of the type including a rigid casing defining a cavity therein and a diaphragm member arranged in the casing to divide the cavity into two chambers, including a gaseous fluid chamber filled with a mass of gaseous spring fluid such as air sealed therein and an oil chamber to be held under oil pressure load.
Conventional forms of such type of spring device have characteristically exhibited a spring constant which increases with increase in amount of load applied to the device over the whole working range thereof, as indicated by the line A in FIG. 5 of the accompanying drawings. In use of the spring device in an automotive suspension system, such spring characteristic is advantageous in that it enables the device to support a heavier load efficiently without impairing riding comfort of the vehicle. However, in the lighter load region, where the spring constant is reduced to an extreme extent, vertical displacement of the vehicle body for any load change is substantially increased and this obviously involves the disadvantage that the vehicle body is subjected to much increased pitching and rolling movements in such load region. In contrast, the line B in FIG. 5 represents a spring characteristic generally required of springs for automotive suspension. In other words, a suspension spring should preferably exhibit a performance characteristic like that of a conventional device in that portion of working range which lies above a certain intermediate load point N and have a spring constant which increases with reduction in load in the load region lower than the point N.
Accordingly, the present invention has for its object the provision of a novel pneumatic spring device of the type described which is adapted to meet the performance requirements described above and simple in construction.
The above and other objects and advantages of the present invention will become apparent from the following detailed description of a few preferred embodiments of the invention as illustrated in the accompanying drawings.